(CNN) -- A man in the medical examiner's office pushed back the cheeks on Baby Grace's corpse to put a smile on her tiny, decaying face. That's how forensic artist Lois Gibson captured the open-mouthed grin with spangled risotto-sized teeth. It was one of the girl's few features that had survived six weeks in a shed during a Texas summer and nearly two months in Galveston Bay.

But it was the way the blond toddler looked outside of the black body bag that made Gibson draw large, lid-filling eyes on the picture of the girl that led to identifying Riley Ann Sawyers.

"She was so very, very small. She looked like the size of a child that you would change the diaper on, just laying there on a metal gurney like a giant stainless steel cookie sheet," Gibson says just above a sad, disbelieving whisper before her voice shifts and she's strong and scientific. "If you are very, very small, then the iris, that colored part of your eye, takes up almost the entire eye opening."

The corpse's decay helped Gibson perfect the gentle downward slope of Riley's eyes. "The decomposition was such that I could see that bone on her face," she said. "And her eyebrows were going to follow her little ridge."

Gibson said she felt her blood pressure rising during her morgue visit. It passed quickly, she said, and "I turned into the artist. I was going to make the best picture possible and get every piece of anatomy right and find her name and get justice for her." In spite of what lay on the gurney, Gibson said, "I knew that she was beautiful, and the picture would reach out to people who knew her and loved her." Watch what the sketch artist has to say about the case »

And it did. Five days after the release of Gibson's precise post-mortem sketch, Riley's grandmother in Ohio recognized the sweet face and contacted police.

It's the kind of result police hope for when they summon one of the handful of forensic artists across the nation who can re-create the faces of maimed, burned and decaying bodies. Gibson spent what she described as just "three intense minutes" with Riley's body that day. It was all she needed. It was all she could take.

But it was decades of research that help her nail the image.

A trained artist can look at a skull and immediately tell the race and gender of a corpse. And that's the beginning of giving a name to the unknown.

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"If you turn the skull of a white male sideways, it's almost like a truck hit it. It's just straight up and down flat," said Marla Lawson, a renowned forensic artist who works for the Georgia Bureau of Investigation. "The forehead protrudes very little and the chin sinks in. But for African-Americans, they slightly slope at the forehead and they protrude slightly at the mandible and they have these great cheekbones. Their skulls weigh more than white people's skulls, and their teeth will be whiter and brighter usually."

An Asian person's skull will have a wider facial area, Lawson said, but people of Hispanic descent are more challenging because the structures are nearly identical to Caucasians.

But determining male and female is easy, said Lawson who created the first, and dead-on, likeness of Olympic Park bomber Eric Rudolph while he was a fugitive. Run your fingers vertically over your eyebrows. Males feel a defined ridge under the brow line. A woman's face is smoother.

There are other clues about structures on a person's head. The size and direction of that really hard bone just behind the ear holds clues about whether earlobes are attached or not, said Suzanne Birdwell, who works for the Texas Department of Public Safety. And those orbits, the openings in a skull where eyes belong, are not all the same in spite of their run-of-the-mill appearance.

"Some part of it will be slanted and that will tell you which way tears ducts would have drained and that tells you where to start the corner of the eye," Birdwell said.

Anthropologists spent years taking measurements from various cadavers -- male, female, chubby, emaciated, short, young, old, Asian, Latino, black, white. They've compiled those measurements in how the depth from skin surface to bone differs based on all those factors for 21 different spots on a face and placed those averages in a chart. Forensic artists use those measurements to determine how much flesh and contour to put on a skull to create a bust or make a sketch.

Frank Bender worked on a case very similar to Riley's in the early 1980's. Bender helped identify Aliyah Davis. Like Riley, she was beaten to death, tossed into a steamer trunk and then dumped under a bridge in Philadelphia, Pennsylvania.

Bender says the best reconstructionists know how to combine science and art because depth charts are limiting. "Facial thickness charts do not make the person. They will give you a gross, generic likeness because they are taken from averages and every person is an individual," he said.

Charts would have failed him when he worked on a case in New York. The front of the skull was gone -- no orbits to determine the slope of eyelids, no hole in the middle to give clues about where the nose would have been or its shape and no upper jaw to hint at lip and mouth shape. But with something that resembled an empty bowl, Bender made a face. The exact face.

He used shapes from the back of the head to determine what kind of face would match the back of that skull. "It's harmony of form," said Bender who is a fine artist by trade. "It's no different than composing a piece music or orchestrating a dance."

Gibson used similar artistic license in painting the case-solving portrait of Riley. Along with the saddening visit to the morgue, Gibson used autopsy photos and pictures of the clothes and shoes on the body.

"I had to go to the morgue to see her in person. I saw how her hair fell to the middle of her back. She looked a lot like my little girl at that age," Gibson said. "I took out a picture of my baby girl playing on the floor with toys at that same age and I drew my daughter's body in that position. And I drew the clothing of the dead girl on that body and I put Baby Grace's face on that body."

Gibson captured the downward slope of Riley's eyes and the tiny nose with the bulbous tip. "I knew I had to do this right for that little girl," Gibson said. "I only get one chance, and I want to help helpless victims. How much more helpless can you be when you're 2, and you're dead."E-mail to a friend